The coastguard was alerted last night that a trawler may have caught an old mine in its nets.
The three men on the boat returned the suspected explosive into the water and came back to shore at Gourock.
Bomb squads were expected to return to the riverbed to examine the ordnance today.
The incident, which happened about 5.40pm, meant the whole area was sealed off, and this caused travel disruption as trains between Glasgow Central and Gourock were delayed or cancelled. Arygll Ferries also suspended crossings between Gourock and Dunoon.
A police spokeswoman said: "As a precautionary measure the ferry terminal was closed. The coastguard is dealing with the incident."
A spokesman from the Clyde Coastguard said: "The ordnance has been taken back out to sea. The Explosive Ordnance Disposal [unit] is going to take a look at it."